Evaluation of high power laser diodes for space applications: effects of the gaseous environment

Transcription

1 Evaluation of high power laser diodes for space applications: effects of the gaseous environment Jorge Piris, E. M. Murphy, B. Sarti European Space Agency, Optoelectronics section, ESTEC. M. Levi, G. Klumel, R. Diamant Semiconductor Devices (SCD), Israel. ICSO 2014, Tenerife

2 Opto-Electronics Laboratories at ESTEC

3 Opto-Electronics Laboratories at ESTEC Support ESA projects and industry Assessments and advice in emergency situations Proof-of-concept experiments Guarantee continuity and build knowledge for future SPACE-PHOTONICS missions

4 Environmental testing of laser diode arrays: Motivation Nowadays, the conventional baseline for the implementation of high energy lasers in space is for them to be contained in an oxygen-rich environment to minimize the effects of laser-induced contamination, which is most dramatic in the UV. Nevertheless the (IR) pump diodes are often tested in different atmospheric conditions we performed a study on the long term effect of the gaseous environment on diode lifetime. During previous qualification campaigns, the question was risen whether or not the effects of mechanical stress would appear immediately or after a burn-in period a group of devices from the long term endurance test was subjected to mechanical shock half way through the environmental long-term test period.

5 Environmental testing of laser diode arrays: The Laser Diode Laboratory at ESTEC Laser diode laboratory facility Appropriate environmental conditions to handle the devices - Clean Room ISO 6, class Molecular contamination control <5 ppb non-volatile hydrocarbons - Environmental stability temperature 21±2 C humidity 50±10% - Emergency power supply

6 Environmental testing of laser diode arrays: The Laser Diode Laboratory at ESTEC Endurance test benches Long term (operational) endurance in space representative conditions The endurance test bench is a rack comprising a series of test containers in which the devices will be operated for extended periods of time under vacuum or contaminant gases while monitoring their electrical and optical parameters.

7 Environmental testing of laser diode arrays: The Laser Diode Laboratory at ESTEC Laser diode carrier Electrical harness and temperature stability Inside each chamber, a carrier hosting 5 LDAs can be inserted. Each position is fitted with TECs to control the base plate temperature, a strip line for the driving current, voltage and temperature sensors.

8 Environmental testing of laser diode arrays: The Laser Diode Laboratory at ESTEC Monitoring Device characteristics and bench common parameters are monitored and recorded for extended periods of time Bench common parameters - Cooling water temperature and stability - Vacuum/gas pressure - Residual gas mass analysis Test position level - Drive current and voltage - Heat sink and diode case temperature - Emission spectrum - Output optical power

9 Environmental testing of laser diode arrays: Test sequence and samples SCD RUBY-9 Diode Type Output peak Power Threshold Current Drive Current Operating Voltage QCW >680 W < 20 A 80 A < 17 A Conversion Efficiency 52 % Center Wavelength Spectral Width (FWHM) 808 nm 4 nm Beam Divergence (FWHM) 35 X 10 Emitting Area Dimensions W-L-H 10 x 2.8 mm 14.4x10.6x.11.6 mm Number of Bars 9 Operating Temperature -24 C to 56 C

10 Environmental testing of laser diode arrays: Output power evolution Output power measured every hour for each device during the test (1000 MShot 50Hz, ca. 300 days) Output power displayed normalised to initial value. Mind different scale on upper plot!

11 Environmental testing of laser diode arrays: Cooling failure A chain of entangled soft and hardware failures left the devices in operation without cooling for 5 days. Reached temperatures ca. 90 instead of the nominal 50 C Output power dropped less than 3% power loss Post-failure power trends similar to those recorded before the interruption The elevated temperatures the devices experienced acted as an accelerated ageing mechanism without further catastrophic consequences. Evidence for robustness of devices Based on these positive observations, it was decided to continue with the test.

12 Environmental testing of laser diode arrays: First half of endurance test Remarkable differences in power loss between the different groups at the half way point of 500 MShots. More than 50% power loss for some devices of the nitrogen group 2-5% decrease in air (3% average) 3-7% in vacuum (6% average) The data suggests that the effects of laser-induced contamination are playing a significant role. A.K.A. Package-Induced Failure: Outgassing introduces carbon deposits on the facets that can yield to thermal runaway and losses of optical power. Proper selection of materials in the package and addition of oxygen reduces these effects. Chambers thoroughly cleaned, contaminants most likely from devices themselves

13 Environmental testing of laser diode arrays: White light interferometry images of bars Device 3 operated in Air Large areas where crystalline surface visible next to others that seem to be (partially) covered by a deposit.

14 Environmental testing of laser diode arrays: White light interferometry images of bars Device 3 operated in Vacuum Thin deposit more obvious than for devices in Air

15 Environmental testing of laser diode arrays: White light interferometry images of bars Device 4 operated in Nitrogen Bar mostly covered, bar surface hardly visible

16 Environmental testing of laser diode arrays: Shock test 2 devices from each group dismounted and subjected to shock on ringing table (2000g for frequencies >2kHz). No visible damage or decrease in power observed during characterization after test. One device from the air group showed increased degradation after 100 MShot, but it was not subjected to shock! We could not correlate the sudden increase in degradation with manipulation/shock as suggested in previous test campaign. Not enough statistics! No signs were found on any characterization technique to anticipate the increases degradation rate at late stage.

17 Environmental testing of laser diode arrays: Detailed characterization of device 1 air group We performed a number of detailed test to identify defects or presence of stress ask me for more info if interested! Near field images (failed amitter distribution) Individual emitter spectra (stress temperature homogenity) Degree of polarization (stress)

18 Environmental testing of laser diode arrays: Second half of endurance test The chambers were flushed and sealed again after the shock test and the long term run resumed. Much slower degradation rate was observed: Nitrogen avrg. 4% vs >50% Air avrg. 0.5% vs 3%(ecluding device 1) Vacuum avrg. 2% vs 6% The first phase acted as an efficient burnin and bake out, in which most contaminants were released: it is possible to condition the devices and reduce the impact of contamination in any atmosphere.

19 Environmental testing of laser diode arrays: Conclusions The data recorded indicate a strong influence of the gaseous environment on the lifetime of the (IR) lasers, suggesting that contamination and out-gassing plays a crucial role in their performance in confined environments. The presence of oxygen seems to palliate these effects, resulting on an extended lifetime even when compared with active vacuum pumping. Pre-conditioning of the devices (vacuum bake-out, oxygen burn in ) could aid palliating contamination effects and allow operation in any desired environment. Most high power laser diode qualification campaigns suffer of low statistics in the number of devices tested, making it difficult to establish causality from correlations. Most important, this reminds us once more the good old lesson: Test as you will fly and fly as you tested

20 Environmental testing of laser diode arrays Many thanks! Laser Lab Team and the Optoelectronics Section at ESTEC And to your for your kind attention!

21 HPLD test facility: Characterization bench Characterization bench Tools for in-depth characterization and analysis Periodically, the carrier containing the devices is extracted from the TeCo and placed without manipulating the devices on a bench where detailed characterization of each device is performed. Example: Initial, mid term and end-of-test extensive electrooptical characterization. Pag. 23

22 HPLD test facility: Characterization bench Monitoring Complete device and single emitter properties can be studied. Voltage and current Integrating sphere - Overall power and pulse energy - Overall spectrum Near field imaging system: emitter resolved images of bars and stacks - Intensity -> failed emitter - Spectrum -> temperature distribution, bar to bar dispersion - Degree of polarization -> presence of stress Pag. 24

23 HPLD test facility: Characterization bench Overall Monitoring Spectra and LIV characteristics Centre wavelength Temperature wavelength dependenc Spectral width Efficiency Threshold current Operating voltage etc. Pag. 25

24 HPLD test facility: Characterization bench Near field imaging system Assessing the contribution of each emitter to the stack overall properties

25 HPLD test facility: Characterization bench Near field imaging system Assessing the contribution of each emitter to the stack overall properties LD stack Beam splitter ND filters Telecentric lens

26 HPLD test facility: Characterization bench Near field imaging system Assessing the contribution of each emitter to the stack overall properties CCD camera 1 Image of intensity distribution Identification of failed emitters Intensity profiles

27 HPLD test facility: Characterization bench

28 HPLD test facility: Characterization bench Near field imaging system Assessing the contribution of each emitter to the stack overall properties Image of degree of polarisation (DoP) DoP Stress -> stress distribution Polariser

29 HPLD test facility: Characterization bench 5A 80A Tensile to compressive Pag. 31

30 HPLD test facility: Characterization bench Near field imaging system Assessing the contribution of each emitter to the stack overall properties CCD camera 2 Image of wavelength distribution Bar to bar wavelength dispersion Spectral shift Temperature -> temperature distribution

31 HPLD test facility: Characterization bench Near field imaging system Direct image Spectra l image

32 HPLD test facility: location, location, location The HPLD test facility is located at ESTEC: access to extensive test facilities and expertise. - Radiation testing - Vibration and shock - Direct access to space qualification experts Pag. 34